In a conventional magneto-optical recording method, recording and reproducing operations on and from a recording medium are performed as described hereinbelow. Here, the recording medium includes a substrate made of glass, plastic, ceramic or other material and a vertically magnetized film formed thereon, which is made of metal magnetic material. In the recording operation, first of all, a magnetization direction of the vertically magnetized film of the recording medium is uniformly arranged to a predetermined direction (upward direction or downward direction) by applying thereon an external magnetic field or the like. (This process is hereinbelow referred to as initialization.) Then a laser beam is projected on a portion of the recording medium where the recording is performed so that a temperature of the portion is raised to a point above the vicinity of the Curie temperature or to a point above the vicinity of the magnetic compensation temperature. As a result, a magnetic coercive force (Hc) at the portion becomes zero or substantially zero. In this state, an external magnetic field (bias magnetic field) having a reverse magnetization direction to the initialized magnetization direction is applied, thereby causing the magnetization direction of the recording medium to be reversed. When the application of the laser beam is stopped, the temperature of the portion illuminated by the laser beam returns to a room temperature and the reversed magnetization direction is thus fixed, thereby permitting information to be recorded. This type of recording is referred to as thermomagnetic recording.
In the reproducing operation, a linearly polarized laser beam is projected onto the recording medium. A polarization plane of a resulting reflected light or transmitted light from or through the recording medium rotates in a different direction according to the magnetization direction of the recording medium (upward direction or downward direction), which is a phenomenon referred to as the magnetic Kerr effect or the magnetic Faraday effect. Information on the recording medium is optically read out by utilizing this phenomenon.
Recording media used in the magneto-optical recording system (magneto-optical disks) have been noted as large capacity memory elements of a rewritable type. Presently, there are two methods for re-writing over the information recorded on the recording medium, as described in (i) and (ii) hereinbelow.
(i) A method wherein, after an erasure of the previously recorded information is performed by initializing the recording medium once again, new information is written thereon.
(ii) A method wherein a recording medium or an external magnetic field generating device is improved so that overwriting can be performed, i.e. the information is re-written directly without performing the erasure.
If method (i) is adopted, either an additional initialization device besides a recording head or two heads for recording and erasing must be installed, thereby causing an increase in the number of parts as well as a rise in cost. Moreover, in case of recording and erasing information with a single head, the same time as required for recording is required for erasing as a waiting time, resulting in the inefficient operation of re-writing information.
In the mean time, if method (ii) is adopted so as to improve the recording medium, it is difficult to control recording medium composition, film thickness and so on. Consequently, the most effective method is to improve the external magnetic field generating device of method (ii), i.e. a magnetic field modulation method for switching a direction of the external magnetic field at high speeds according to information to be recorded, while keeping the intensity of the laser beam constant.
In order to switch the direction of the external magnetic field at high speeds, a magnetic head (i.e. a coil and a coil core) of the external magnetic field generating device must be miniaturized to a great degree. In this case, however, a generating area of the magnetic field becomes smaller. In order to counteract this, a magnetic head and a recording medium must be brought closer to each other. As shown in FIG. 10 and FIG. 11, generally a floating head 1 of a sliding type which can glide over a recording medium in the shape of a disc (not shown) is employed as the external magnetic field generating device. The floating head 1 is provided with a slider section 2 and a magnetic head section 3 installed in the slider section 2. Further, a suspension 4 composed of plate springs and other members, having its one end fixed to a base 5, supports the floating head 1 secured to the other free end thereof so that it is pressed down toward the recording medium. According to the configuration, when the recording medium is rotated, the floating head 1 floats over the recording medium with a small amount of gap from its surface.
A constant floating height of the floating head 1 is maintained due to the fact that an upward floating force caused by the air flowing between the slider section 2 and the recording medium balances with a downward depressing force by the suspension 4. The floating head of this type is also used in conventional hard disk devices. In the case of the hard disks, the floating height is of a submicron order. When a magneto-optical disk is used as the recording medium, a floating height of 5 .mu.m to 15 .mu.m is necessary, i.e, a larger floating height is required for the magneto-optical disk than for the hard disk. The reasons for this are as follows. Since the magneto-optical disks are transportable, dust tends to stick more frequently on the disk. As a result, a too small floating height might cause troubles such as a head crash where the magnetic head section 3 is damaged by dust.
In the case of a magneto-optical disk which requires such a large floating height of 5 .mu.m to 15 .mu.m, variations in the floating height depend greatly on the relative speed between the floating head 1 and the recording medium. For example, when the recording medium is rotated based on the constant angular velocity control system, the relative speed increases as the floating head 1 moves toward the circumferential portion of the recording medium, and consequently the floating height also increases.
As described above, the relative speed between the floating head 1 and the recording medium varies depending on a radial position of the recording medium, and this causes the floating height of the floating head 1 with respect to the recording medium to vary. In this case, the magnitude of a magnetic field to be applied to the recording medium varies depending on a radial position of the recording medium. This causes a problem that recording and erasing operations cannot be performed under constant conditions.
A surface of the magneto-optical disk facing the bottom surface of the floating head 1 is textured with fine and physical protrusions and recessions (hereinafter, referred to as texture), thereby preventing these surfaces from sticking to each other. As shown in FIG. 12(a) and FIG. 12(b), when forming the texture on the surface of the magneto-optical disk, a texture tape 15 having fine protrusions and recessions on the surface thereof is depressed onto a rotating magneto-optical disk 14 by a tape pressure roll 16. Then, the texture tape 15 is on the one hand fed in a direction indicated by an arrow C, and on the other hand moved from the circumferential portion to the inner portion of the magneto-optical disk 14 while being depressed thereon by the tape pressure roll 16. In this case, a rotation direction of the magneto-optical disk 14 is substantially parallel to a feeding direction of the texture tape 15. Therefore, as shown by concentric lines in FIG. 13, substantially following the rotation direction of the magneto-optical disk 14, a texture is uniformly formed on the surface of the magneto-optical disk 14 facing the floating head 1.
However, in the texture forming method as described above, it is necessary to provide a process for depressing the texture tape 15 on each one of the magneto-optical disks 14. For this reason, the method described above results in high manufacturing costs of the magneto-optical disks 14, thereby failing to provide a suitable method for quantity production. Further, another problem is presented in that when forming the texture, the quality is lowered due to unnecessary stress imposed on the magneto-optical disk 14 or dust produced during the process that adheres to the magneto-optical disk 14.